The present invention relates to a voice circuit of an electronic telephone.
Recently, electronic telephones have been put into practical use in place of telephones in which a channel is formed by a transformer. A voice circuit of such an electronic telephone is constituted as shown in, e.g., FIG. 1. Namely, a voice signal input from a transmitter 11 is amplified by an amplifier 12 and then supplied to a line driver 13. Line driver 13 controls a take-in amount of a DC bias current flowing between a pair of telephone lines 14a and 14b in accordance with a level of the voice signal input from transmitter 11, thereby transmitting a signal corresponding to the voice signal input from transmitter 11 onto telephone lines 14a and 14b. Signals which are transmitted through telephone lines 14a and 14b are amplified by an amplifier 15 and thereafter they are supplied to a receiver 16 and output as a voice signal therefrom.
In the voice circuit of such an electronic telephone, the operating power source voltages of amplifiers 12 and 15 and the like are produced from the currents flowing through telephone lines 14a and 14b by use of a regulating power supply circuit 17. Power supply circuit 17 is constituted by, for example, connecting a resistor 18 and a capacitor 19 in series between telephone lines 14a and 14b. The voltage having a value corresponding to the voltage drop due to resistor 18 is subtracted from the DC voltage between telephone lines 14a and 14b and supplied as operating power source voltage to amplifiers 12 and 15.
In such a voice circuit, an amplifier, for example, amplifier 15, when used to drive receiver 16, consumes a large amount of current. In this case, regulating power supply circuit 17 needs to supply a high voltage and a large current. For this purpose, the resistance of resistor 18 needs to be set at a low value.
An AC impedance with respect to telephone lines 14a and 14b has a value which is almost equal to an impedance of resistor 18. Therefore, if the resistance value of resistor 18 is reduced to enable regulating power supply circuit I7 to supply a high voltage and a large current as mentioned above, the AC impedance of the telephone will decrease. However, in general, the AC impedance of the telephone has a rated value. If the AC impedance is set to a low value as mentioned above, it will be hard to match the AC impedance of the telephone with this rated value. Moreover, in the case where the AC impedance of the telephone doesn't satisfy the rated value, there is a possibility that the capability of the system (exchanger) to transmit the reception signal deteriorates.
However, if the resistance value of resistor 18 is set to increase the AC impedance of the telephone end, at the same time, if it is set to realize the supply of a high voltage by regulating power supply circuit 17, then the DC impedance must be set to a high value. In such a situation, in the case where a conventional telephone is used, for example, one in which the channel is constituted by a transformer, and is connected in parallel with the same telephone lines 14a and 14b and made operative simulaneously with the electronic telephone, a sufficient operating power source voltage is not supplied to the electronic telephone due to the difference between the DC impedance values, resulting in unsatisfactory operation of the electronic telephone.